Balancing Equations

Law of Conservation of Mass• States matter can neither be created

nor destroyed.• Thus, atoms are neither created nor

destroyed, only rearranged in a chemical reaction.

• So…the number of a particular atom is the same on both sides of a chemical equation.

How Molecules are Symbolized

• Molecules may also have brackets to indicate numbers of atoms. Ex: Ca(OH)2

• Notice that the OH is a group• The 2 refers to both H and O

Cl2 2Cl 2Cl2

O HO

HCa

Subscripts vs. Coefficients• The subscripts The subscripts

tell you how tell you how many atoms of many atoms of a particular a particular element are in element are in a compound. a compound. The coefficient The coefficient tells you about tells you about the quantity, the quantity, or number, of or number, of molecules of molecules of the compound.the compound.

How many?• How many of each atom are in the

following?

a) NaOHb) Ca(OH)2

c) 3Ca(OH)2

Na = 1, O = 1, H = 1Ca = 1, O = 2, H = 2Ca = 3, O = 6, H = 6

Chemical Equations• Their Job: Depict the kind of Their Job: Depict the kind of

reactantsreactants and and productsproducts and their and their relative amounts in a reaction.relative amounts in a reaction.

• 4 4 AlAl (s) + 3 (s) + 3 OO22 (g) ---> 2 (g) ---> 2 AlAl22OO33 (s) (s)

• The letters (s), (g), (aq) and (l) are The letters (s), (g), (aq) and (l) are the physical states of compounds.the physical states of compounds.

Steps to Balancing Equations

1. Write the correct chemical equation (if not already given to you). DO NOT TRY AND BALANCE IT YET!

2. Count the number of atoms of each element on the left side and compare to the number of atoms of each element on the right.

3. Determine where to place coefficients so that the number of atoms of each element is the same on the left and right.

4. Check to make sure the entire equation is balanced.

Balancing Equations• When balancing a chemical

reaction you may add coefficients in front of the compounds to balance the reaction, but you may notnot change the subscripts.– Changing the subscripts changes the

compound. Subscripts are determined by the valence electrons (charges for ionic or sharing for covalent)

Some Helpful Hints• LEARN THESE DIATOMIC MOLECULES: O2 H2 N2 F2 Cl2 Br2 I2

• Take one element at a time, working left to right.

• Leave O and H to the end.

• IF everything balances except for O, and there is no way to balance O with a whole number, double all the coefficients and try again.